Reland "[gn build] port d1d36f7 (llvm-tapi-diff)"

This reverts commit d84ed15
The original change relanded in 5b5ab80.

Author: nico

llvm/utils/gn/secondary/llvm/test/BUILD.gn

@@ -273,6 +273,7 @@ group("test") {
     "//llvm/tools/llvm-split",
     "//llvm/tools/llvm-strings",
     "//llvm/tools/llvm-symbolizer:symlinks",
+    "//llvm/tools/llvm-tapi-diff",
     "//llvm/tools/llvm-undname",
     "//llvm/tools/llvm-xray",
     "//llvm/tools/lto",

llvm/utils/gn/secondary/llvm/tools/llvm-tapi-diff/BUILD.gn

@@ -0,0 +1,11 @@
+executable("llvm-tapi-diff") {
+  deps = [
+    "//llvm/lib/Object",
+    "//llvm/lib/Support",
+    "//llvm/lib/TextAPI",
+  ]
+  sources = [
+    "llvm-tapi-diff.cpp",
+    "DiffEngine.cpp",
+  ]
+}